Metal-Enhanced H-Bond Donor Tendencies of Urea and Thiourea toward Anions: Ditopic Receptors for Silver(I) Salts

The heteroditopic receptors 1 and 2 firmly include an AgI ion into the NS2O2 crown and establish selective H-bond
interactions with anions at the covalently linked urea/thiourea (U/T) subunit. Studies in MeCN solution showed that
the metal center induces a 103-106-fold enhancement of the anion association constant, due to a coordinative
interaction with the O/S atom of the U/T moiety. In the presence of F-, the Ag-O interaction promotes the release
of a proton from one of the urea N-H fragments. AgI coordination to the S atom of the thiourea derivative 2
promotes the formation of unusually stable H-bond complexes with Cl- and Br-.